In a fluid catalytic (cat) cracking unit, catalyst is circulated in transfer lines between two vessels, a reactor and a regenerator; each of which can be provided with a dense phase fluidized bed of the catalyst. The feed, generally a gas oil, is preheated and injected usually with recycle feed, into one of the circulating lines which contains hot regenerated catalyst which is being moved, or transported, from the regenerator to the reactor. The catalyst vaporizes the fresh feed and recycle feed, and the vapor with steam is injected to atomize the liquid feed, and fluidize the catalyst. The feed is cracked at about 900.degree. F. in the fluidized bed of the reactor in an endothermic reaction which consumes the heat brought from the regenerator by the catalyst. Coke is deposited on the catalyst, this inevitably lessening its activity. The cracked products from the reactor are passed through a staged cyclone separator system which collects entrained catalyst fines and returns them to the reactor. The vapors are passed to a primary fractionator, which separates the gasoline and lighter products from the heavy products. The gasoline and lighter products are sent to a light ends recovery unit while the heavy material is separated and recycled to the reactor, or withdrawn from the unit.
Regeneration of the catalyst by the removal of deposited coke, and heat balance of the unit are both essential; and in commercial operations both are interrelated. Spent catalyst is withdrawn from the bottom of the reactor, stripped with steam, and then recycled, or conveyed by a stream of air into the regenerator where deposited coke is burned from the catalyst in a dense phase fluidized bed at temperatures ranging from about 1100.degree. F. to 1400.degree. F. Entrained catalyst is removed by staged cyclone separators, and flue gas is removed from the stack. The hot regenerated catalyst is withdrawn from the regenerator and transported to the reactor to supply heat to the latter.
The spent catalyst recycled to the regenerator contains some unburned hydrocarbons, since it is not possible by present techniques to remove all of the hydrocarbon vapors from the catalyst by stripping. Hence, some hydrocarbon products are carried from the reactor to the regenerator and burned; this consuming some of the regeneration air, and decreasing the yield of useful products. In the combustion, it is desirable to burn all of the coke in the regenerator to carbon dioxide; which, if possible, is certainly not practical. Carbon monoxide is also formed. Carbon monoxide in the flue gas, however, represents wasted heat values, except to the extent that the carbon monoxide can be burned in downstream equipment for recovery of the wasted heat. The flue gases leaving the regenerator differ in composition depending upon the part of the regenerator bed the gases leave since there is only a small amount of lateral mixing of the gases ascending through the regenerator bed. Variations in the carbon monoxide-containing flue gas composition leaving the regenerator can be troublesome. This is because the flue gas is often burned with air in a steam generator, or in a furnace which directly warms the fresh feed or heats other refinery streams. Variations in the carbon monoxide content of the flue gas creates undesirable process instability.
It is desirable to burn all the carbon monoxide in a cat cracker regenerator to very low values, i.e., 500 ppm or less, to meet air polution regulations directly, or to conserve all the heat of combustion in the process. In this case it is desirable to have as good a gas distribution as possible inasmuch as it reduces the amount of excess oxygen required to meet the desired low carbon monoxide level. For example, a unit with poor solids/gas distribution may require an excess oxygen content of 2 to 3 percent to obtain a 500 ppm carbon monoxide level, while a unit that has good solids/air distribution can achieve the 500 ppm level with only 0.5 percent excess air.
Some compensation for this unequal distribution of gases in the regenerator bed might be provided by introducing additional air at the location where the catalyst enters the vessel. However, this results in high local gas velocities in the bed which does little to dispose of the problem, and in fact may introduce new problems.
It is, accordingly, the primary objective of the present invention to provide improved apparatus, and process whereby the velocity distribution of air is equalized throughout the bed.
A more specific object is to provide improvements in a regenerator which will divert the gases entering into the dense phase fluidized bed of said regenerator outwardly, and more evenly, this producing more uniform flow of spent coked catalyst into the regenerator bed and the generation of a more uniform flue gas composition throughout the regenerator.
These objects and others are achieved in accordance with the present invention embodying apparatus and process wherein spent, coked catalyst is admixed with combustion air externally to the regenerator and introduced through an annulus, annular area or zone around the spent, coked catalyst standpipe from which regenerated catalyst is removed from the regenerator and recycled to the reactor. Generally from about 3 percent to about 20 percent, preferably from about 5 percent to about 10 percent, of the total air introduced into the regenerator is introduced in this manner and the balance of the air, which is in itself sufficient for complete combustion, is introduced via a separate air inlet into the regenerator. This produces a more even flow of the spent, highly coked catalyst into the regenerator with the result that the coke is burned from the catalyst and a flue gas of more constant composition is produced throughout the regenerator. Hydrocarbon contaminants from the reactor, not removed by stripping, are also effectively burned and as a result, the flue gas generated in this portion of the catalyst bed differs little from flue gas formed in other parts of the bed.
The invention, and its principle of operation, will be better understood by reference to the following more detailed description, and to the attached drawings, to which reference is made in the description.